Breast cancer metastasis to lungs, liver, bone and brain is the primary cause of death in breast cancer patients. It involves cancer cell dissemination via the blood stream and depends on adhesive and invasive tumor cell functions and their ability to survive and proliferate at the target site. Bogenrieder et al., Oncogene 22: 6524-6536, 2003. These events are supported by integrins, a family of transmembrane adhesion receptors composed of α and β subunits. Felding-Habermann, Clin. Exp. Metastasis 20: 203-213, 2003; Hood et al., Nat. Rev. Cancer 2: 91-100, 2002. Integrins exist in distinct states of activation, which determine the affinity for ligand and regulate whether soluble ligands are bound, which matrix proteins are recognized, and the degree to which cells can adhere, migrate and arrest under dynamic flow conditions as found in the circulation. Tadokoro et al., Science 302: 103-106, 2003; Shattil et al., J. Clin. Invest. 100: 1-5, 1997; Felding-Habermann, et al., Proc. Natl. Acad. Sci. U.S.A 98: 1853-1858, 2001.
An integrin found on breast cancer cells, but not on normal breast epithelium is αvβ3. Expressio[n of this receptor correlates with invasion and cancer progression. Liapis et al., Diagn. Mol. Pathol. 5: 127-135, 1996; Pignatelli et al., Hum. Pathol. 23: 1159-1166, 1992. Human breast cancer cells can express αvβ3 in an activated or a non-activated functional state. Activated αvβ3 supports breast cancer cell attachment during blood flow, and strongly promotes invasive tumor cell migration. In particular, only the activated state supports target organ colonization by circulating breast cancer cells in a mouse model, and metastatic cells isolated from breast cancer patient blood express αvβ3 in a constitutively activated form. Felding-Habermann et al.; Proc. Natl. Acad. Sci. U.S.A 98: 1853-1858, 2001; Rolli et al., Proc. Natl. Acad. Sci. U.S.A 100: 9482-9487, 2003. Integrin transition between distinct states of activation is associated with conformational changes within the heterodimer. Calzada et al., J. Biol. Chem. 277: 39899-39908, 2002; Beglova et al., Nat. Struct. Biol. 9: 282-287, 2002; Xiong et al., Science 294: 339-345, 2001; Xiong et al., Science 296: 151-155, 2002; Pampori et al., J. Biol. Chem. 274: 21609-21616, 1999.
Like other integrins, αvβ3 can exist in distinct states of activation and functional affinity. The activated or high affinity state of αvβ3 has a unique molecular conformation that is distinct from the non-activated state. Xiong et al., Science, 294: 339-345, 2001; Xiong et al., Science, 296: 151-155, 2002; Xiong et al., Blood, 102: 1155-1159, 2003.
In contrast to the non-activated receptor, activated αvβ3 is functionally characterized primarily through its ability to bind soluble ligand proteins, to support tumor cell interaction with platelets during blood flow and thereby mediated tumor cell arrest under conditions as found in the vasculature, and to promote invasive tumor cell and endothelial cell migration very strongly. The latter is probably very important for angiogenesis. The ability of the activated αvβ3 integrin to bind soluble ligands is a key property that enables the ligand-mimetic scFv antibodies to bind specifically and exclusively to the activated conformation of αvβ3.
No therapy is known today that prevents cancer, for example, breast cancer, from becoming systemic, and there is little understanding of even how to design and test such drugs; yet metastases ultimately are responsible for much of the suffering and mortality from breast cancer. A need exists to identify and target molecular and functional markers that identify metastatic breast cancer cells and to generate reagents for their specific inhibition.